!974 USA Spec twin Stromberg problem

[Redderd]

Hi all,

I have a 1974 USA spec TR6 with twin Stromberg's that i have just rebuilt with new jets, spindles,seals etc. I've put them back on the car and although i can get a steady idle if I rev the car the revs won't drop back to idle. they will drop to about 1200 RPM and from there may even rise to 2000 or so.

it's not the linkages sticking as with the throttle linkage disconnected i can rev either card independently and get the same result.

Pistons appear to move freely without sticking if i raise them and then let them drop.

on occasion i also hear a "fluttering" sound from the front carb.

Appreciate lots of you have PI but I thought I'd ask if anyone ideas or experienced these symptoms before?

Cheers

Dave

 

[harlequin]

Dave

I would double check that the spindles are not sticking and that you dont have an air leak, and then when you are happy all is correct suspect the temperature compensators or bypass valves especially if you have had them apart.

Don't try adjusting either before reading the excellent article on the buckeye web site as I found it very difficult to get the settings right (I actually permanently disabled both and the car now runs really well )

As a quick check the temperature compensators can be temporarily disabled by putting a bit of sticky tape over the port to see if that improves things.

George

[poolboy]

When you have the throttle shafts disconnected and each carb is independent of the other, see if pushing down on the throttle stop lever makes the idle drop back to normal..

Try each carb separately.

If it does return to normal idle speed, the problem is that the throttle shaft seal is putting a bind on the throttle shaft...If you followed the Buckeye technique for replacing the seals that's a real possibility.

Anyway, keep us updated; we can figure it out.

Here is a picture of the throttle stop lever. It is seen on the lower left side of the yellow background:

Edited February 11, 2017 by poolboy

[oldtuckunder]

At idle and above a single stromberg will feed all 6 cyl using the balance tube so the problem may only affect one carb.

 

Poolboy's suggestion isn't a bad one, as if the rod at the back of the stop lever is stuck in it can bleed air into the inlet manifold from a couple of sources.

 

With the US spec carbs (and whilst a few denigrate the temperature compensator and bypass valve) when set up properly they are a major advance over the earlier models.

 

So a few things you can try after checking the above rod is free to move

 

On the filter side of the carb just below and to the left of the right hand filter mounting bolt you will see a hole. Block these off maybe one at a time and then both. This will stop all air flow to both the temperature compensator and the bypass valve, if this cures the problem then one of those is at fault.

 

Also as you say its a USA spec, I assume that you are actually in the UK :-) so the big question is what did you do with all the emmisions control pipework? If its still in place then you may have a lot more digging to do. However the easiest thing to do is if you can see the piece of pipe to the right of Poolboy's picture block them off, don't link them between the two carbs with a piece of pipe, just block them off.

 

OK if you have them blocked off, and the two ports that feed the temperature compensator and bypass valve, and you still have the problem then you have got something wrong with the rebuild or installation. If the problem goes away then you know where to start looking. NB common failures with the temperature compensator are missing the sealing O ring around the port that goes into the carb, quite often missing so you wouldn't know to replace them, and a sticking valve in the compensator, they get gummed up and can set in place. Whip them off, remove the cover, give them a good clean with carb cleaner and then test in boiling water, you can dip them in and out of an alternating hot/cold pots of water and you should see the valve move, if it doesn't its stuck, they usually respond to cleaning well.

 

Once you get them set up they will stay set for ages without going out of tune, and will be way better than crappy HS SU's (diving for cover)

 

Alan

[poolboy]

At idle and above a single stromberg will feed all 6 cyl using the balance tube so the problem may only affect one carb.

 

Poolboy's suggestion isn't a bad one, as if the rod at the back of the stop lever is stuck in it can bleed air into the inlet manifold from a couple of sources.

 

 

 

On the filter side of the carb just below and to the left of the right hand filter mounting bolt you will see a hole. Block these off maybe one at a time and then both. This will stop all air flow to both the temperature compensator and the bypass valve, if this cures the problem then one of those is at fault.

 

. However the easiest thing to do is if you can see the piece of pipe to the right of Poolboy's picture block them off, don't link them between the two carbs with a piece of pipe, just block them off.

 

 

 

Alan

 

Alan you still have a lot to learn about those carbs...

Dave, whatever you do DO NOT cap off that nipple "piece of pipe" as Alan told you...It's important to either have it plumbed to the Carbon Canister OR left open to atmosphere....trust me.

AND, blocking the "hole" in the carbs air box mounting flange that Alan described has no affect on the Throttle Bypass Valve..

It will however block any air that may pass thru an OPEN temperature compensator.

As far as what I suggested...it's really pretty simple. If the throttle stop lever can be pushed down, it's because the throttle shaft is in a bind from the throttle shaft seal which is preventing the throttle disc from returning to it's idle position.

 

Start with my suggestion....please.

If it fails to give results, there are other possibilities, I can probably help with but start with the simple stuff first.

Edited February 12, 2017 by poolboy

[oldtuckunder]

 

 

Alan you still have a lot to learn about those carbs...

Dave, whatever you do DO NOT cap off that nipple "piece of pipe" as Alan told you...It's important to either have it plumbed to the Carbon Canister OR left open to atmosphere....trust me.

AND, blocking the "hole" in the carbs air box mounting flange that Alan described has no affect on the Throttle Bypass Valve..

It will however block any air that may pass thru an OPEN temperature compensator.

As far as what I suggested...it's really pretty simple. If the throttle stop lever can be pushed down, it's because the throttle shaft is in a bind from the throttle shaft seal which is preventing the throttle disc from returning to it's idle position.

 

 

 

As I suspect most of us do!

 

Sorry about the pipe capping off suggestion, in my haste I thought it was a picture of the crankcase/carbon canister purge pipe on the other side of the carb which are a regular source of air leeks, and should be capped off for testing.

 

However also for "testing" it would do absolutely zero harm to block off the vent pipe, as all this would do is stop vapours from the float chamber venting to the charcoal canister when the throttle is shut. As a permanent fix I drill a 3/16 hole through the vent pipe and through the brass bush, and then cap the vent pipe off. That way the float chamber vents to the air filter at all times and the operation/position of the rod from throttle stop lever becomes irrelevant.

 

Yes also need to apologise, blocking off the feed to the temperature compensator will not block off the feed to the bypass valve, but will block off the feed to the fine mixture adjustment screw which can also be a source of air leeks.

 

Re bypass valve operation, I guess it depends if you have the earlier version with an external vacuum pipe to actuate it, or the later one that relies on inlet manifold vacuum and tend to be more reliable.

 

Yes your suggestion to test the closure of the throttles first is correct, however I would point out that your suggestion to push down on the stop lever would have the opposite effect and would in fact open the the throttle a bit, Ah but as I write I realise the confusion, you meant push down on the top of throttle leaver to ensure it was closed, whereas I think of the vent lever as being the stop lever as its the one with the adjustable stop!

 

I had foolishly concluded that as the operation of the piston drop had been tested, that simple closure of the throttle plates had also been, and that we were looking for something more subtle.

 

Alan

[poolboy]

FWIW the Throttle Bypass Valves as found on the 250, 69 and 74 TR6's with the external nipple, rely on manifold vacuum to activate...same vacuum source as the 70, 71, 72, 73, 75 and 76 versions of the carb albeit from a less direct route.

And it would NOT be a good idea to block, plug or otherwise close the nipple that vents the float chamber when the engine is idling.

I can't stress that enough..

Capping that nipple, which is found only on the 73 thru 76 carbs, has caused problems enough so that I made this picture and colored the nipple red if there were any doubts as to it's location.

 

As far as the permanent so called "fix" ....that's irrelevant and would be of no help whatsoever to Dave which is what I'm trying to do, not compare notes.

Edited February 12, 2017 by poolboy